1. Field of the Invention
The present invention generally relates to a reversible piezochromic system, and methods of making and using the reversible piezochromic system.
2. Discussion of the Related Art
Reversible chromic devices have a wide variety of applications. They have been heavily employed especially in the field of display technology. The devices are generally able to convey a change in color in response to a specific stimulation.
Reversible chromic devices have for example been known to exist in complicated electrical and mechanical devices. An example of these types of devices includes well known liquid crystal displays. Such displays employ the manipulation of the orientation of liquid crystal materials by application of an electric or electromagnetic field. These types of devices generally require a power source and tend to be heavy, expensive and difficult to operate at times.
Alternatives to these devices have been developed with the use of electrochromic, photochromic, thermochromic or piezochromic devices. In electrochromic devices, the chemical change is induced with the application of an electromagnetic field. In photochromic devices, the change is induced by exposure to light. In thermochromic, devices the change is caused by a change in temperature. In piezochromic devices, by contrast, the change is induced by pressure.
Common to these systems, however, is their dependency on the internal chemical change that occurs within a given substance. This makes these types of devices less desirable in reversible applications as they cannot be easily or accurately controlled.
For example, in piezochromic devices, the color of a piezochromic substance is attributed to the absorption of light in specific regions of the excitation spectrum of the substance which causes a jump of an electronic transition from a ground state to a state of higher energy. When pressure is applied to the substance, these energy states are disturbed. By so doing, a change in color of the substance is noticed. An example of this type of device is described in WO 2005/092995, which discloses reversible piezochromic system used in the field of printing inks to protect banknotes from forgery. The system comprises a combination of an electron donating compound and an electron accepting compound. The electron donating compound is an ionochromic substance such as a pH sensitive dye. The electron accepting compound exhibits an acidity that is high enough to develop a color change, but low enough to allow for a reversible color change. Although very useful for some applications, this type of application also presents some issues relating to fast fading of the color (e.g., one or two minutes). Such relaxation times are undesirable in the field of forgery or monitoring devices.
WO 2010/115928 also describes a reversible piezochromic system used for forgery protection. The system includes optically contrasting pigment particles oriented in a film or a coating of an elastic polymer. When the elastic polymer is compressed or elongated, the density and orientation of the pigment particles change to produce a visible color change. One of the drawbacks of this device is the inability to control the duration of the color change.
The Piezochromic effect has also been studied with respect to liquid crystals in devices without the use of electrical components. Here, the molecules of a liquid crystal are disposed on a sheet or between two substrates. When pressed upon, a disruption of the liquid crystal's alignment causes a color change. The produced color, however, does not exhibit a high contrast ratio, and the relaxation time for the color to revert back to its equilibrium state remains very difficult to control.
Reversible devices often times are necessary in more simple applications. They must not only operate properly, they must also be easy and inexpensive to manufacture. The use of color changing chemical compositions, or electrical devices is less desirable in such circumstances due to their complexity and cost.
An exemplary, alternative to such costly devices was introduced in U.S. Patent Publication No. 2009/0211642 which describes a monitoring device that uses an observable fluid. The fluid moves between a first and second reservoir to indicate a change in color. However, difficulties in signal detection may exist because all of the fluid must drain from one reservoir to another reservoir until the fluid is no longer visible in the drained reservoir. Thus, relying on pressure to remove all of the fluid may be insufficient. Also, the device tends to be large in view of its two reservoirs and would not be suitable in smaller devices. Further, manufacturing of the device is challenging in view of its complex structure.
For at least these reasons, a need exists in the art for an improved, reversible piezochromic system that can easily be manufactured and is relatively inexpensive, but that can still provide an appropriate level of control over its functionality.